Telephone service to a highly significant number of customer-users is supplied by telephone companies on a party-line basis. Such multiple customer usage of a singular line of communication is particularly prevelant in small towns, as well as in rural areas and the like where line distances from a switching station to individual telephone users are of extensive length.
Because of the power losses necessarily encountered with such lengthy transmission lines, the number of customers served or parties available to a given party line is limited, particularly with respect to the number of telephone instrument ringer components involved. The latter components perform in conjunction with a ringing system, the basic function of which is to ring a called party, give ring-back tone to the calling party, and halt ringing the ring-back tone when the call is answered or abandoned. The called party's ringer component is driven from alternating current ringing generators and in the present-day approach to telephone design, the ringer components are designed to ring only over certain frequency and voltage ranges.
Such multi-frequency ringing systems serve to replace "coded" (short-long, etc.) ringing on multi-party lines. Typical ringer components are structured as mechanically actuated bells having a clapper of selected spring bias and ring frequency sensitizing components including a capacitor-inductor arrangement which provides a frequency tuned clapper drive response. Generally, a sequence of predetermined frequency levels is utilized by the telephone companies to carry out ringing to a given party on a party line. Commonly, five distant frequencies are employed in allowing individual party ringing for up to five parties on a given line with bridged ringing and up to ten parties with divided ringing. Harmonic, decimonic and synchromonic series ringing systems generally are employed in the telephone service industry. The harmonic series are 16.6, 25, 33.3, 50, and 66.6 Hertz. Decimonic series are provided as 20, 30, 40, 50 and 60 Hertz, while the synchromonic series are 16 (or 20), 30, 42, 54, and 66 Hertz. Telephone service companies commonly carry 12 telephone models in their repertory which will be capable of responding to 12 different ringing frequencies. As is apparent, such an arrangement requires the provision of 96 items in inventory, which from a business standpoint, is less than a desirable requirement.
Telephone service companies also have experienced a wide range of difficulties with the ringer components of ringing systems, particularly with respect to frequency selective ringers. A satisfactory ring by one of these components is usually considered as a steady two-gong ring or a certain level of sound output expressed in dBrap. A small change in input frequency can cause a considerable increase in minimum voltage required for a ring. At some point in an altering frequency, the ringer component will not respond, even at maximum ringing voltage. Because of the nature of the delivery systems from switching stations, distortion or deviation from a proper sinusoidal input will affect the performance of the ringers. While a frequency selective ringer component will perform at a particular input frequency, as this frequency is increased or decreased, a higher voltage is needed to produce a comparable ring. In effect, the difficulty in maintaining frequency selectivity, and the loads imposed by the mechanical ringers importantly limit the number of parties which are available on any given line. Ringer components constitute the major part of this ringing load or power demand. Further, achieving desired frequency selectivity on the part of ringers on an efficient high volume production basis has been an elusive goal of industry. The mechanical bell ringers have been subject to very high reject rates even before their installation within telephone sets. Part of this rejection difficulty involves the carrying out of tuning procedures with the coils and capacitors operatively associated in a typical mechanical bell ringer device. To the present time there are no electronic ringers which are frequency selective in performance perhaps due to requirements for a stable performance and a capability for repeating in response to frequency inputs on a regular basis from unit to unit. The ringers also are prone to other operational difficulties. For example, ringers have a sensitivity to 20 Hertz ringing appear prone to exhibit a phenomena known in the industry as "dial tap". Dial tap results from the spike from the collapsing magnetic field of the pulsing relay of the system and causes a "ding" with each dial pulse and/or with disconnect. To overcome a dial tap condition, the above-noted spring bias of the clappers of ringers is stiffened with the expected result that higher power is required to cause proper ring.
From the foregoing it will be apparent that a frequency selective ringer circuit which exhibits very low power consumption while a remaining highly discriminating with respect to frequency inputs would be of particular utility to the industry. Of further interest to the telephone service industry would be such a ringer circuit or component which could be accurately tuned for any of the above ranges of frequency response by the simple expedient of closing a switch. Such a simplified tuning procedure will eliminate the difficulties encountered in attempting the tuning of current mechanical bell systems. Further, industry will be receptive to such devices where they can be installed by the customer without resort to telephone company labor input.